Method and apparatus for auto-exposure control in the presence of artificial illumination

ABSTRACT

A method and apparatus is disclosed for auto-exposure control in a scene under artificial illumination that synchronizes the exposure control algorithm and final image capture with the intensity variations in the artificial illumination.

RELATED APPLICATIONS

[0001] This application is related to the following application: “Colorcorrection for a scene based on the detection of artificial illuminationin the scene” that has the H.P. docket number 10015227, “A method andapparatus for detecting the presence of artificial illumination in ascene” that has the H.P. docket number 10016239, and “A method andapparatus for auto-focus control in the presence of artificialillumination” that has the H.P. docket number 100110429. All threeapplications were filed on the same day as this application.

FIELD OF THE INVENTION

[0002] The present invention relates generally to auto-exposure controland more specifically to a method and device that can auto-expose in thepresence of artificial illumination in a scene.

BACKGROUND OF THE INVENTION

[0003] When capturing an image with a digital camera, the source of theillumination for the scene affects the colors captured with the camera.For indoor scenes the illumination source can vary widely and caninclude a tungsten bulb, halogen lamps, fluorescent lamps, sunlightcoming in through a window, or even a xenon light. Each of these typesof light sources has a different spectral energy distribution. The typesof light sources that create light using a filament glowing at a hightemperature (for example tungsten bulbs) are typically characterized bya color temperature defined as a Planckian radiator with a temperatureof 50 degrees higher than the filament of the light. The sun can also becharacterized as a Planckian radiator but the loss of some wavelengthsthrough scattering and absorption in the atmosphere causes significantdifferences from the Planckian radiator at those wavelengths. Because ofthe variation in the spectral power distribution of the sun, standardspectral power distribution curves have been developed. One of thestandard curves is called D65 corresponding to a color temperature of6500 K. Clouds in the sky can also affect the spectral distribution ofenergy reaching the scene from the sun. The time of day also affects thecolor temperature of the sun (noon vs. sunrise). The color temperaturecan be affected by whether the object is in direct sun light or inshadows.

[0004] The types of light sources that excite a phosphor layer that thenfluoresces (for example fluorescent lamps and xenon lamps) tend to havespectral distributions that are unique to the phosphors in the lamp incombination with the mercury vapor spectrum.

[0005] Each of these light sources has a different spectral powerdistribution that affects the colors captured in a scene by a camera.For example when you have a white object illuminated by a tungsten bulbthe white object will appear yellow in the scene captured by the camera.This is because the tungsten bulb does not produce much blue light. Awhite object is an object that reflects a similar amount of the red,green and blue light that hits the object. When a white object isilluminated by a tungsten bulb more red light is hitting the object thanblue light and therefore more red light is reflected, causing the objectto look yellow to the camera. The human eye adjusts to differentilluminates and compensates for the color shift but a camera records theactual light in the scene.

[0006] Fortunately these color shifts caused by the illumination sourcecan be corrected. This correction is typically called white balancing.For proper white balancing the illuminant of the scene must be known.There are a number of methods currently used to try to determine thescene illuminant to be used in white balancing.

[0007] One method looks for the brightest point in a scene and assumesthat it should be white. The brightest point is then adjusted until itis white and then this adjustment is used to balance the rest of thescene. This method operates on the assumption that the brightest pointin a scene is from a white object or from a specular reflection. Forexample the specular reflection coming from a car windshield. Obviouslynot all scenes have the brightest point as a specular reflection or awhite object. When this method is used on a scene with a non-whiteobject that is the brightest point in the scene it can result insignificant color mismatch. Another method of white balancing adjuststhe image until the sum of all the areas in the image adds up to aneutral gray. Both of these methods operate on assumptions about thecontent of the scene.

[0008] Another method uses a correlation matrix memory to map the imagedata onto color image data under a number of different illuminants. Thismethod is described in U.S. Pat. No. 6,038,339 “White pointdetermination using correlation matrix memory” inventers Paul M. Hubelet al. that is hereby incorporated by reference. When using this methodthe image data needs to be mapped onto the color data for all potentialilluminants. Mapping the image data onto each of the potentialilluminants is a computational process. If the set of potentialilluminants could be narrowed to the type of illuminant (for exampledaylight) the amount of computation, and therefore the time could bereduced. One way to narrow the set of potential illuminants is todetermine if the scene contains artificial illumination. Therefore theability to detect the presence of artificial illumination can increasethe speed and accuracy of the color correction algorithms inside digitalcameras.

[0009] Typically most artificial illumination sources are powered byalternating current. There are two main frequencies for alternatingcurrent. The United States uses 60 Hz and Europe uses 50 Hz. At thesespeeds, the human eye typically does not detect variations in thebrightness of the artificial illuminant. However, digital cameras andother devices that detect light using today's photo sensors can and dodetect the variation in brightness due to the alternating current (AC)driving most artificial illumination sources. The brightness variationtypically is larger under fluorescent illumination sources and smallerunder incandescent illumination sources. These variations in intensitycan cause problems for some of the automatic functions in digitalcameras like auto-focus and auto-exposure.

[0010] When using the auto-exposure function, the camera adjusts thelens aperture, the exposure length and gain of the photo sensor togather the correct amount of light for a proper exposure. Theauto-exposure function relies on accurate measurements of the amount oflight within the scene to set the exposure parameters. The exposurelengths for photo sensors, typically a CCD, when measuring light for theautomatic-exposure function has a typical range from {fraction (1/60)}to {fraction (1/1000)} of a second. Exposure measurement errors can belarge if the exposure lengths are smaller than the period of the drivingfrequency of the AC power source. When scene illumination varies becauseof artificial illumination, incorrect final image exposure may result ifthe variation in intensity is not taken into account.

[0011] When using the auto-focus function, the camera adjusts theposition of the lens to focus the scene on the photo sensor. Typicallycameras use a measure of contrast between areas in the scene todetermine proper focus. The auto focus algorithm typically takesmultiple exposures of a scene with the lens in different positions, andthen selects the lens position corresponding to the exposure with thehighest contrast. Unfortunately the level of illumination in the sceneaffects the contrast in a scene. This can result in a highfocus-contrast measurement during a bright part of the artificial lightsource cycle and a low focus-contrast measurement during a dimmer partof the light source cycle. If the light is brighter during anout-of-focus focus-contrast measurement, the out-of-focus position maybe chosen as best unless this variation in intensity is taken intoaccount.

[0012] Therefore there is a need for a system that can determine thepresence of artificial illumination in a scene and compensate for thevariations in intensity.

SUMMARY OF THE INVENTION

[0013] A method and apparatus for auto-exposure control in the presenceof artificial illumination in a scene is disclosed. By matching thesampling rate or exposure length for the auto-exposure algorithm withthe frequency or period of the driving AC current, the variations inintensity of the artificial illuminant can be accounted for.

[0014] Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a chart of the variation in intensity of an artificialilluminant powered by alternating current.

[0016]FIG. 2 is a chart of the variation in intensity of an artificialilluminant powered by alternating current sampled using an exposurelength not equal to the period of the AC frequency.

[0017]FIG. 3 is a chart of the variation in intensity of an artificialilluminant powered by alternating current sampled using an exposurelength equal to the period of the AC frequency.

[0018]FIG. 4 is a chart of the variation in intensity of an artificialilluminant powered by alternating current sampled using an exposurelength much smaller than the period of the AC frequency.

[0019]FIG. 5 is a chart showing a waveform sampled at a differentfrequency than the waveform.

[0020]FIG. 6 is a flowchart where the amount of variability indicatesthe type of light in a scene.

[0021]FIG. 7 is a chart showing one embodiment of the current inventionwhere the final exposure is centered at a cross-over point in the phaseof the variation in intensity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] A method and apparatus that can compensate for the presence ofartificial illumination in a scene can improve digital cameras and otherdevices that capture scenes using photo sensors.

[0023] Artificial illumination is typically powered by alternatingcurrent. There are two main frequencies for alternating current. TheUnited States uses 60 Hz and Europe uses 50 Hz. The alternating currentdriving artificial illumination causes the intensity of the illuminationto vary at twice the driving frequency. The intensity variation isdependent on the type of artificial illumination. Incandescent lightstypically have smaller intensity variations than fluorescent lights. Theintensity variations typically follow a fluctuating variation at twicethe rate of the sinusoidal variation in the alternating current (seeFIG. 1). These variations in intensity can cause problems for theauto-exposure controls used in digital cameras. When artificialillumination is detected in a scene, the auto-exposure algorithm can beadjusted to compensate in ways such as setting the exposure length to Ndrive periods or by synchronizing the exposures to the phase of thedriving source.

[0024] There are many different ways that the presence of artificiallight can be determined. One way is by user input. In one embodiment ofthe current invention the user is asked if the scene is an indoor sceneor if the scene is illuminated by artificial light. The user could alsoindicate the AC frequency used for powering the artificial lights. Someusers may be unaware of the frequency of AC current used. However mostusers know which country they are in. If the user indicates they are inthe U.S. then the frequency can be determined to be 60 Hz, and if theuser is not in the U.S. then the frequency would typically be 50 Hz. AGPS device could also be used to determine the location of the deviceand therefore determine the AC driving frequency. Another way is to usean electronic circuit that is capable of sampling the AC power sourcefrequency when the device is powered from an AC source.

[0025] Another way to detect the presence and driving frequency ofartificial illumination is by sampling the light in the scene anddetermining if there are periodic intensity variations. Photo sensorstoday, typically charged coupled devices (CCD), can change the timebetween exposures (sample rate) as well as exposure lengths.

[0026] In one embodiment of the current invention the exposure length isadjusted such that the exposure length is not equal to the period ormultiple of any of the common AC frequencies. The two most common ACfrequencies are 60 Hz and 50 Hz therefore the two most commonillumination periods are {fraction (1/120)} second and {fraction(1/100)} second. An example exposure length that is not equal to theperiod of these two AC frequencies is {fraction (1/140)} second, this isjust an example and many other exposure lengths could be used. A numberof exposures are taken using this exposure length. The sample rate ortime between exposures is not critical but should not match any of theexpected AC frequencies. The overall brightness of the scene iscalculated for each exposure, using methods well know in the arts, forexample averaging the light for all the pixels in the scene. The overallbrightness for each exposure is compared for variability betweenexposures. Because the exposure length is different than the AC periodthe average intensity of light during the exposure will be differentdepending on the phase of the driving AC at the start of the exposure(see FIG. 2). When the exposure starts at time 202 the AC is fallingtoward its minimum and the average light intensity 204 during theexposure will be low. When the exposure time starts at time 206 the ACis starting to reach its peak 208 and the average light intensity duringthe exposure will be higher 210. These changes in average lightintensity will be detected as variability in the average brightnessbetween the multiple exposures taken. If there is low variability thenthe amount of artificial illumination in the scene is low. If there ishigher variability then the amount of artificial illumination in thescene is high. The variability in the overall brightness can be comparedto a threshold value and when the variability is higher than thethreshold the scene contains an artificial illuminant.

[0027] Once the presence of artificial illumination has been detectedthe frequency of the AC can be determined. The exposure length isadjusted to match the period or a multiple of the period of one of thecommon AC frequencies. A number of exposures are taken and the averagebrightness of the scene for each exposure is once again calculated. Whenthe exposure length matches the period of the AC frequency thevariability between the exposures will be reduced (see FIG. 3). Thevariability is reduced because wherever the exposure starts the fullperiod of the driving AC is included in the exposure and the averagelight intensity is the same. Exposure 302 starts as the phase is nearingits peak and has average intensity of 304. Exposure 306 starts as the ACis reaching the cross over point and has an average intensity of 308.There is low variability between level 304 and 308 therefore theexposure length must match the period of waveform 300. Table 1 shows thevariability in scene brightness for fluorescent lights at 50 Hz and 60Hz and sunlight. TABLE 1 Light source type - AC frequency Exposurelength Variability Artificial - 60 Hz AC 1/(60*2) 17 Artificial - 60 HzAC 1/(50*2) 426 Artificial - 50 Hz AC 1/(60*2) 293 Artificial - 50 Hz AC1/(50*2) 5 Sunlight 1/(60*2) 11 Sunlight 1/(50*2) 7

[0028] If the variability is still high the process is repeated with adifferent exposure length until an exposure length is found that reducesthe variability. The exposure length that reduces the variability willbe the period of the driving AC frequency.

[0029] In another embodiment the first exposure length is chosen tomatch a period of one of the common AC frequencies, for example 60 Hz.Multiple exposures are taken and the variability between exposures iscalculated. The sample rate or time between exposures is not criticalbut in the preferred embodiment would be an integer multiple of theexposure time. If there is high variability an artificial illuminant ispresent and the process is repeated with a different exposure length todetermine the frequency of the driving AC. If there is low variabilityit could be because of two reasons. It could either be caused by havinglittle or no artificial illumination in the scene or it could be causedby the AC period matching the exposure length. This can be determined bychanging the exposure length to match a different AC frequency than thefirst exposure length. Using the second exposure time a number ofexposures are taken and the variability in brightness between exposuresis calculated. A low amount of variability indicates a low amount ofartificial illumination in the scene. If the variability is now higherthen artificial illumination is present in the scene and the artificialillumination is being driven at the frequency that the first exposurelength was matched with.

[0030] In another embodiment of the present invention the exposurelength is chosen to be smaller than the period of any of the common ACfrequencies. In the preferred embodiment the exposure length would bemuch smaller than ½ the smallest period of any of the common ACfrequencies. 60 Hz has a light intensity fluctuation period of {fraction(1/120)} of a second, ½ of that is {fraction (1/240)}l of a second.Therefore in the preferred embodiment the exposure length would be{fraction (1/480)}^(th) of a second or shorter. Using this shortexposure length multiple exposures would be taken with a sample ratethat does not synchronize phase with light fluctuations from any of thecommon AC frequencies. The overall brightness of each exposure would becalculated and the variability in brightness between the differentexposures would be calculated. Because the time between exposures isdifferent than the AC period the average intensity of light during theexposure will be different depending on the phase of the driving AC atthe start of the exposure (see FIG. 4). When the exposure starts at time402 the AC is starting to reach its peak and the average light intensity404 during the exposure will be high. When the exposure time starts attime 406 the AC is starting to reach the cross over point 408 and theaverage light intensity during the exposure will be lower 410. Thesechanges in average light intensity will be detected as variability inthe average brightness between the multiple exposures taken. Highvariability indicates the presence of artificial illumination. FIG. 5 isa chart showing the results of sampling a waveform at a differentfrequency than the waveform. Once artificial illumination is detected inthe scene the frequency and the phase of the variation in intensity canbe determined.

[0031] General sampling theory states that to determine the frequencyand phase of a waveform the sampling rate must be at least twice thefrequency of the waveform (the Nyquist limit). However determining thefrequency and phase of a waveform that is constrained to a fewwell-known frequencies of a known shape, for example a sine wave, doesnot require sampling at twice the frequency. This is because thereflections of the base frequency and the harmonics of the basefrequency are used to differentiate between the few expectedfrequencies. Analyzing a sampled waveform using Fast Fourier Transforms(FFT), and discarding the frequency results that don't match the fewcommon AC frequencies, allows the frequency and phase of the lightvariations to be determined.

[0032] Another way to determine the frequency is to adjust the start ofeach of the exposures to be synchronized in phase with one of the commonAC frequencies and then recording the brightness for a number ofexposures. This process is repeated with other common frequencies untilthe variability of the average light intensity between exposures isfound to be smaller at one frequency than the other. The reducedvariability occurs because the average intensity of each sample will beapproximately the same when each exposure starts at the same place onthe waveform. Once the frequency has been determined the phase can bedetermined by moving the starting exposure time along the period of thewaveform while looking for minimum or maximum brightness levels in themeasured light.

[0033] In another embodiment of the present invention the exposurelength is chosen to be smaller than the period of any of the common ACfrequencies. In the preferred embodiment the exposure length would bemuch smaller than ½ the smallest period of any of the common ACfrequencies. 60 Hz has a light intensity fluctuation period of {fraction(1/120)} of a second, ½ of that is {fraction (1/240)} of a second.Therefore in the preferred embodiment the exposure length would be{fraction (1/480)}th of a second or shorter. Using this short exposurelength multiple exposures would be taken using a sample rate that wasmatched to one of the common AC frequencies. The overall brightness ofeach exposure would be calculated and the variability in brightnessbetween the different exposures would be calculated. If there is highvariability an artificial illuminant is present and the process is canbe repeated with a different sampling rate to determine the frequency ofthe driving AC. If there is low variability it could be because of tworeasons. It could either be caused either by having little or noartificial illumination in the scene or it could be caused by the ACperiod matching the sampling rate. This can be determined by changingthe sampling rate to match a different AC frequency than the firstsampling rate. Using the second sampling rate a number of exposures aretaken and the variability in brightness between exposures is calculated.A low amount of variability indicates a low amount of artificialillumination in the scene. If the variability is now higher thenartificial illumination is present in the scene and the artificialillumination is being driven at the frequency that the first samplingrate was matched with.

[0034] In another embodiment of the current invention the contrast inthe scene is used instead of the overall brightness level in the sceneto determine the presence of artificial illumination. Scene contrast istypically used in camera auto-focus algorithms. There are many differentways, well known in the arts, to calculate scene contrast. One way is totake the difference in intensity between adjacent pixels. Because scenecontrast is dependent on overall levels of scene illumination,variations in scene illumination can be detected by changes in scenecontrast. Scene contrast is also dependent on how well focused the sceneis onto the photo sensor. When the scene is well focused, changes inscene brightness can be more easily detected using scene contrast thanwhen the scene is poorly focused. In the preferred embodiment when usingscene contrast, the scene is focused onto the photo sensor with a lensbefore the detection for artificial illumination proceeds. In oneembodiment using scene contrast, short exposure lengths are used and thesampling rate is chosen such that it does not match any common ACfrequencies. Multiple exposures are taken and the overall contrast ineach exposure is calculated. The variability in contrast between thedifferent exposures is then calculated. High variability in the contrastbetween the exposures indicates the presence of artificial illumination.The variability is in general proportional to the amount of variation inthe light source. The amount of variation in brightness and averagebrightness of the scene may be correlated to the type of light source. Afluorescent light source typically has a higher variability in contrastthan an incandescent light source. When the variability is smaller thana first threshold (604) there is little, if any, artificial illuminationin the scene (608). When the variability is larger than the firstthreshold but smaller than a second threshold (610) the variabilityindicates incandescent illumination (612). And when the variability islarger than the second threshold (614) the variability indicatesfluorescent illumination (616).

[0035] When artificial illumination is detected in a scene the contrastmeasurements can be re-done using a sample rate that corresponds to oneof the common AC frequencies. If the variation between the contrastmeasurements decreases then the correct AC frequency has beendetermined.

[0036] In another embodiment using contrast measurements with shortexposure lengths, the sample rate is picked to match one of the commonAC frequencies. If there are large variations in the contrastmeasurements then artificial illumination is present in the scene beingdriven at a different frequency than the sample rate. If the variationin contrast measurements are low, a second series of measurements ismade at a second sampling rate corresponding to another common ACfrequency. If the variation in contrast measurements for the second setof exposures are also low, then there is little artificial illuminationin the scene. If the variability of the second set of contrastmeasurements is high, then there is artificial illumination in the scenebeing driven at the first AC frequency.

[0037] Once the presence and the driving frequency of artificialillumination have been determined the auto exposure algorithm cancompensate for the intensity variations. One way to compensate is toadjust the exposure length used in determining the exposure parameters.

[0038] In one embodiment the exposure length is set to the period or amultiple of the period of the intensity variation. Because the exposurelength contains a full period of the intensity variation the averageintensity will be constant independent of where the exposure starts onthe phase of the intensity variation. For example, when the driving ACis at 60 Hz the intensity variations are at twice that frequency.Therefore the period of intensity variations would be {fraction (1/120)}second. When the auto-exposure control takes an exposure using anexposure length equal to {fraction (1/120)}^(th) of a second or aninteger multiple of that length, the intensity variations would notaffect the measurements. Once the proper exposure parameters have beendetermined a final exposure is taken. If the final exposure length isnot equal to the period or an integer multiple of the period of theintensity variations, the final exposure should be centered at thecrossover point of the intensity variations (see FIG. 7). When anexposure is centered at the crossover point 706 the average intensity isindependent of the exposure length. Exposure length 702 is longer thanexposure length 704, but both exposures have the same average level ofintensity 708 because they are both centered at the crossover point inintensity variations 706.

[0039] In some cases exposure lengths that are different than the periodof the intensity variations are desired for the auto-exposurecalculations. When the exposure length used in the auto-exposure controlare different than the period of the intensity variations the timingbetween multiple exposures used for the auto-exposure calculations andthe final exposure must be controlled.

[0040] In one embodiment the exposure lengths used in the auto-exposurecalculations are kept constant. The exposures are synchronized with thefrequency of the intensity variations in the scene. The exposures can besynchronized at the same frequency, an integer multiple of thefrequency, or an integer divisor of the frequency of the intensityvariations. For example, when the intensity variations have a frequencyof 120 Hz, the exposures used in the auto-exposure calculations can besynchronized at 120 Hz, 240 Hz or 60 Hz. These are just three of themany potential frequencies that could be used for synchronization inthis example. The starting place or phase on the intensity variation isunimportant. The exposure length is also unimportant. Because eachexposure starts at the same place or phase of the intensity variationsand the exposures are the same length there will be little variation inintensity between exposures. To minimize the intensity variations in thescene the final exposure length should match the exposure lengths usedin the exposure calculations and the final exposure starting time shouldbe synchronized to the same place used in the auto-exposurecalculations. In another embodiment the final exposure is centered atthe crossover point of the intensity variations. In this embodiment thefinal exposure length can be different than the exposure length used inthe auto-exposure calculations.

[0041] In another embodiment the exposures used in the auto-exposurecalculations are timed to be centered at the cross-over point in theintensity variations 706. In this embodiment the exposure lengths do notneed to be the same. The final exposure is also centered at thecrossover point of the intensity variations. Because the final exposureis also at the cross-over point the final exposure length can bedifferent than the auto-exposure lengths.

[0042] In another embodiment of the current invention the location ofthe scene is determined. The location may be determined by user input orthe location may be determined by a GPS device or the like. Typicallythe location does not need to be precise, in most cases only the countryneeds to be determined. In some countries, for example Japan, both 50 Hzand 60 Hz are present. In countries where multiple AC frequencies areused another method may be preferred. With the location of the scenedetermined the frequency of the AC current used at the scene locationmay be assumed. For example when the scene is located in the U.S. thefrequency of the AC current would be assumed to be 60 Hz. In thisembodiment the test for the presence of artificial illumination is notdone. The exposure length used in the auto-exposure control is adjustedto match an integer period of ½ of the assumed frequency. The finalexposure length should also be matched to an integer multiple of ½ ofthe period of the assumed frequency. In this embodiment theauto-exposure control will correctly determine the exposure parametersindependent of the presence or absence of artificial illumination in thescene.

[0043] The foregoing description of the present invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and other modifications and variations may be possible inlight of the above teachings. The embodiment was chosen and described inorder to best explain the principles of the invention and its practicalapplication to thereby enable others skilled in the art to best utilizethe invention in various embodiments and various modifications as aresuited to the particular use contemplated. It is intended that theappended claims be construed to include other alternative embodiments ofthe invention except insofar as limited by the prior art.

What is claimed is:
 1. A method for auto-exposure control, comprising:determining a scene location; setting an exposure length equal to aninteger multiple of ½ a period of the AC current typically used at thescene location; taking at least one exposure of the scene using theexposure length; determining at least one exposure parameter for thescene using the at least one exposure.
 2. The method of claim 1 wherethe scene location is determined by user input.
 3. The method of claim 1where the scene location is determined by a GPS device.
 4. The method ofclaim 1 where the exposure parameter comprises an exposure length. 5.The method of claim 1 where the exposure parameter comprises an aperturesize.
 6. The method of claim I where the exposure parameter comprises again factor.
 7. A method for auto-exposure control, comprising:determining a presence of artificial illumination in the scene;determining a frequency of intensity variations in the scene;synchronizing an exposure rate with the frequency of intensityvariations in the scene; taking at least one exposure of the scene atthe synchronized exposure rate; determining at least one exposureparameter for the scene using the at least one exposure.
 8. The methodof claim 7 where the presence and frequency of the artificialillumination is determined by user input.
 9. The method of claim 7 wherethe presence and frequency of the artificial illumination is determinedby measuring the light from the scene for periodic changes.
 10. Themethod of claim 9 where the periodic changes are variations inbrightness.
 11. The method of claim 9 where the light from the scene isfocused onto a photo sensor and the periodic changes are variations incontrast.
 12. The method of claim 7 where the frequency of theartificial illumination is determined by the geographic location of thescene.
 13. The method of claim 7 where the exposure parameter comprisesan exposure length.
 14. The method of claim 7 where the exposureparameter comprises a gain factor.
 15. The method of claim 7 where theexposure parameter comprises an aperture size.
 16. The method of claim 7further comprising: taking a final exposure, using the exposure setting,at the synchronized exposure rate.
 17. The method of claim 7 furthercomprising: taking a final exposure, using the exposure setting, wherethe final exposure is centered at a cross-over point in the intensityvariations.
 18. A method for auto-exposure control, comprising:predicting at least one frequency for a variation in the illumination inthe scene; measuring light from the scene at a periodic rate, where theperiodic rate is different than any of the predicted frequencies, usingan exposure length that is different than any of the periods of thepredicted frequencies; detecting the presence of an artificialilluminant when the measured light from the scene contains periodicchanges; determining the phase and frequency of the periodic changeswith FFT analysis of the sampled light; synchronizing an exposure ratewith the frequency of the intensity variations in the scene; taking atleast one exposure of the scene at the synchronized exposure rate, theat least one exposure having an exposure length; determining at leastone exposure parameter for the scene using the at least one exposure.19. The method of claim 18 where the exposure length is centered at acrossover point in the intensity variations.
 20. The method of claim 19where a final exposure is taken, using the exposure parameter, and thefinal exposure is centered on a crossover point in the intensityvariations.
 21. A method for auto-exposure control, comprising:predicting a frequency for a variation in the illumination in the scene;measuring light from the scene at a periodic rate using a first exposurelength that is equal to the period of the predicted frequency;re-measuring light from the scene at a periodic rate using a secondexposure length that is equal to the period of a second predictedfrequency; determining the presence and frequency of the variation inthe illumination in the scene when the variability of the measurementsusing the first exposure length is different than the variability of themeasurements using the second exposure length; synchronizing an exposurerate with the frequency of the intensity variations in the scene; takingat least one exposure of the scene at the synchronized exposure rate,the at least one exposure having an exposure length; determining atleast one exposure parameter for the scene using the at least oneexposure.
 22. An apparatus for auto-exposing a scene comprising: a meansfor measuring light from the scene at a periodic rate using apredetermined exposure time; a means for determining the presence andfrequency of intensity variations from an artificial illuminant byexamining the measured light from the scene for periodic intensityvariations; a means for determining exposure parameters for the scenesynchronized with the frequency of intensity variations.
 23. A digitalcamera comprising: a photo sensor array, the photo sensor arrayconfigured to measure light from a scene at a periodic frequency using apredetermined exposure length; a lens configured to focus the light fromthe scene onto the photo sensor array; a processor, the processorconfigured to determine the frequency of intensity variations in theillumination of the scene by examining the measured light from the scenefor periodic contrast variations, the processor also configured tosynchronize at least one exposure, used in an auto-exposure control, tothe intensity variations in the scene.
 24. The digital camera of claim23 where a final exposure is taken synchronized to the intensityvariations in the scene.
 25. A method for auto-exposure control,comprising: determining a presence of artificial illumination in thescene; determining a period of intensity variations in the scene;setting an exposure length equal to an integer multiple of the period ofthe intensity variations in the scene; taking at least one exposure ofthe scene using the exposure length; determining at least one exposureparameter for the scene using the at least one exposure.
 26. The methodof claim 25 where the presence and period of the artificial illuminationis determined by user input.
 27. The method of claim 25 where thepresence and period of the artificial illumination is determined bymeasuring the light from the scene for periodic variations.
 28. Themethod of claim 27 where the periodic changes are variations inbrightness.
 29. The method of claim 27 where the light from the scene isfocused onto a photo sensor and the periodic changes are variations incontrast.
 30. The method of claim 25 where the period of the artificialillumination is determined by the geographic location of the scene. 31.The method of claim 25 where the exposure parameter comprises anexposure length.
 32. The method of claim 25 where the exposure parametercomprises a gain factor.
 33. The method of claim 25 where the exposureparameter comprises an aperture size.
 34. The method of claim 25 furthercomprising: taking a final exposure, using the exposure setting andusing the exposure length.
 35. The method of claim 25 furthercomprising: taking a final exposure, using the exposure setting, wherethe final exposure is centered at a cross-over point in the intensityvariations.